The secret letters Wallis Simpson wrote her former husband

The scandalous marriage of Wallis Simpson and Edward VIII, who famously gave up the throne for the woman he loved, is regularly referred to as "the love story of the last century."

While there have been rumours over the years that Wallis had "cold feet" before her marriage to Edward, there's never been any real evidence — until now.

Edward was the only British sovereign to voluntarily abdicate, preferring to be cast aside from the royal family than to spend the rest of his life without the love of his life by his side. In 1936, Edward stepped down from the throne. He'd been king for only 10 months.

Seventy years after their marriage, a series of letters emerged that gave an insight into what Wallis was going through in the weeks preceding her marriage and the first few years of her marriage to Edward.

Wallis Simpson pictured with Edward, the Duke of Windsor in the Bahamas, circa 1942. (Getty)

What makes this story so fascinating is that several of the letters, quite literally a long-forgotten treasure trove, were written by Wallis to her former husband, Ernest Simpson.

Wallis's first wedding was in November 1916, to US navy aviator Earl Winfield Spencer, who was known as "Win." Win was said to be a heavy drinker and the relationship was incredibly stormy – the couple broke up for several months before reuniting and breaking up again.

Wallis was said to have had a long-term affair with Argentine diplomat Felipe de Espil, and eventually divorced Win in 1927. Interestingly, during her marriage Wallis witnessed two fatal airplane crashes, giving her a lifelong fear of flying.

Wallis married her second husband, Ernest Simpson, in July 1928 and for several years the marriage appeared to be thriving. Ernest was born in the US but educated in the UK, serving in the British army during WWI before becoming a shipbroker in his family's company.

Controversially, Wallis was still legally married when she began a relationship with Edward VIII. (Getty)

At the time he fell in love with Wallis, Ernest was married with a daughter, but left his family to start a new life with the popular socialite.

Seven years into the marriage, Wallis met Edward and, when they started their affair, Wallis was still legally married to Ernest. Their divorce wasn't finalised until just weeks before Wallis remarried.

Even following their divorce, Wallis and Ernest always remained friends.

Wallis sent Ernest flowers when he was in hospital for surgery, and Ernest regularly offered advice when Wallis was working on her autobiography.

The former couple also kept up their tradition of writing long, revealing letters to each other that remain historically important today.

As she was researching her biography of Wallis Simpson, That Woman: the life of Wallis Simpson, historian Anne Sebba discovered an intriguing collection of 15 letters written by the socialite to her former husband, Ernest.

Wallis Simpson and the Duke of Windsor on their wedding day, June 1937. (AAP)

The letters began around the time the couple divorced through to the first two years of Wallis's life in France, where she was living in exile with Edward.

What the letters reveal is a rush of affection between the divorced couple. It was a relationship that had previously been left out of the official biographies and historical documentaries about Wallis.

Sebba has claimed the letters came from an "unnamed source" who had held onto them for decades. But others believe the letters had been deliberately concealed so that no further damage could have been done to Edward's reputation.

Whatever the reason the person had for holding onto the letters, Sebba couldn't believe what she was seeing: written in Wallis's distinctive large, round handwriting, Sebba felt as though she was holding dynamite.

The majority of the letters are postmarked 1936 and 1937.

Some believe Wallis' letters to her ex husband were hidden to prevent damage to Edward's reputation. (Getty)

Sebba writes: "As I read on in utter amazement at the intimate tone of these 15 unpublished letters, written to a man she was supposed to hate, I discovered a desperately unhappy woman terrified of being physically attacked – she was receiving alarming letters threatening her life, including bomb warnings – and full of anxiety about her future."

In 1936, not long after her wedding, Wallis wrote to Ernest, "I think of us so much, though I try not to. I wonder so often how you are? How the business is getting on etc. I thought I'd write a few lines to say I'd love to hear from you if you feel like telling me a bit.'

She wrote about having a "good side and bad side" that were always at war.

"The dual side of my nature will out and you filled my one side so utterly. Anyway I shall always be struggling with myself to the grave and whereas other people will become happy I shall never be able to answer either of my sides satisfactorily. If only one of me was stronger than the other."

Wallis also let Ernest know she wasn't happy that he was romantically involved with her friend Mary Kirk, whom Ernest married six months after he and Wallis were divorced.

"Never a day goes by without some hours thought of you," Wallis wrote in one letter to her ex husband. (Getty)

Wallis wrote that Mary had "left a wound that will never heal," adding: "I shall write about it again. It is very painful and it is too late. Wherever you are, you can be sure that never a day goes by without some hours thought of you and for you and again in my prayers at night. With love, Wallis."

Wallis also wrote to Ernest during her famous trip to Germany, where she shook hands with Hitler.

"Wherever you are, you can be sure that never a day goes by without some hours' thought of you," she writes.

Interesting note: both Wallis and Ernest referred to Edward, rather condescendingly, as "Peter Pan."

Sebba wrote that Wallis's letters reveal a woman who clearly blamed the newly-created Duke of Windsor for "this mess" (when writing to Ernest) adding, "Oh dear, wasn't life lovely, sweet and simple."

One biographer believes Edward and Wallis' love story was "a tale of gothic darkness". (Getty)

Days before the abdication in December 1936, Wallis wrote to Ernest:

"I really can't concentrate on anything at the moment, my dear. I've had so much trouble and complications with everyone. Also, I'm terrified of the court etc. I feel small and licked by it all. I can't think of what sort of mess I'm leaving for. I'm sorry for myself, I'm sorry for the King. I hate stuffy British minds and, last but not least, I don't understand myself, which is the cause of all the misery. Give me courage. I'm so lonely 2:15 Tuesday. love Wallis."

Sebba, who has read all 15 letters, believes they reveal that, perhaps, the marriage was once heralded the "most romantic love story of the last century" was even more complex than we might have imagined.

Instead, Sebba describes the union as being more aptly described as "a tale of gothic darkness."